Figure 1. Percentage Changes in Risk of Developing
Obesity Among Nonobese Women and in Risk of Developing Type 2 Diabetes Among
Nondiabetic Women Associated With Television (TV) Watching, Other Sedentary
Behaviors, and Walking

A, Adjusted for age, smoking, alcohol consumption, and dietary covariates.
B, Adjusted for age, smoking, alcohol consumption, family history of diabetes,
and dietary covariates. All sedentary behavior variables are included simultaneously
in the model. Other sitting includes reading, mealtime, and at desk. Error
bars indicate 95% confidence intervals.

Context Current public health campaigns to reduce obesity and type 2 diabetes
have largely focused on increasing exercise, but have paid little attention
to the reduction of sedentary behaviors.

Objective To examine the relationship between various sedentary behaviors, especially
prolonged television (TV) watching, and risk of obesity and type 2 diabetes
in women.

Design, Setting, and Participants Prospective cohort study conducted from 1992 to 1998 among women from
11 states in the Nurses' Health Study. The obesity analysis included 50 277
women who had a body mass index (BMI) of less than 30 and were free from diagnosed
cardiovascular disease, diabetes, or cancer and completed questions on physical
activity and sedentary behaviors at baseline. The diabetes analysis included
68 497 women who at baseline were free from diagnosed diabetes mellitus,
cardiovascular disease, or cancer.

Main Outcome Measures Onset of obesity and type 2 diabetes mellitus.

Results During 6 years of follow-up, 3757 (7.5%) of 50 277 women who had
a BMI of less than 30 in 1992 became obese (BMI ≥30). Overall, we documented
1515 new cases of type 2 diabetes. Time spent watching TV was positively associated
with risk of obesity and type 2 diabetes. In the multivariate analyses adjusting
for age, smoking, exercise levels, dietary factors, and other covariates,
each 2-h/d increment in TV watching was associated with a 23% (95% confidence
interval [CI], 17%-30%) increase in obesity and a 14% (95% CI, 5%-23%) increase
in risk of diabetes; each 2-h/d increment in sitting at work was associated
with a 5% (95% CI, 0%-10%) increase in obesity and a 7% (95% CI, 0%-16%) increase
in diabetes. In contrast, standing or walking around at home (2 h/d) was associated
with a 9% (95% CI, 6%-12%) reduction in obesity and a 12% (95% CI, 7%-16%)
reduction in diabetes. Each 1 hour per day of brisk walking was associated
with a 24% (95% CI, 19%-29%) reduction in obesity and a 34% (95% CI, 27%-41%)
reduction in diabetes. We estimated that in our cohort, 30% (95% CI, 24%-36%)
of new cases of obesity and 43% (95% CI, 32%-52%) of new cases of diabetes
could be prevented by adopting a relatively active lifestyle (<10 h/wk
of TV watching and ≥30 min/d of brisk walking).

Conclusions Independent of exercise levels, sedentary behaviors, especially TV watching,
were associated with significantly elevated risk of obesity and type 2 diabetes,
whereas even light to moderate activity was associated with substantially
lower risk. This study emphasizes the importance of reducing prolonged TV
watching and other sedentary behaviors for preventing obesity and diabetes.

Current public health campaigns to reduce obesity and type 2 diabetes
have largely focused on increasing exercise levels, but have paid little attention
to the reduction of sedentary behaviors. Television (TV) watching is a major
sedentary behavior in the United States. In a survey conducted in 1997, an
adult male spent approximately 29 hours per week watching TV, and an adult
female spent 34 hours per week.1 In recent
decades, in parallel with increasing obesity, there has been a steady increase
in the number of homes with multiple TV sets, videocassette recorders (VCRs),
cable TV, and remote controls, as well as the number of hours spent watching
TV.1

Compared with other sedentary activities such as sewing, playing board
games, reading, writing, and driving a car, TV watching results in a lower
metabolic rate.2 Constant exposure to food
advertising leads to increased food and calorie intake and unhealthy eating
patterns.3- 5 It
is well established that prolonged TV watching is associated with obesity
in children.6- 8 However,
the role of TV watching compared with other sedentary behaviors, such as sitting
at work or reading, in the development of obesity and type 2 diabetes among
adults has not been well studied, especially among women. We therefore examined
prospectively the relationship between several common sedentary behaviors
and incidence of obesity and type 2 diabetes in a large cohort of women.

METHODS

Study Participants

The Nurses' Health Study cohort was established in 1976 when 121 700
female registered nurses aged 30 to 55 years and residing in 1 of 11 US states
responded to mailed questionnaires regarding their medical history and health
practices; details have been published elsewhere.9 For
this study, we excluded women with diagnosed cardiovascular disease (n = 3102),
cancer (n = 9917), or diabetes (n = 4373) in 1992. The final sample for the
diabetes analysis consisted of 68 497 women. For the obesity analysis,
we also excluded women who were already obese (body mass index [BMI] ≥30)
in 1992 or earlier cycles, leaving 50 277 women in the final analysis.

Assessment of Sedentary Behaviors and Physical Activity

In 1992, participants were asked to report their average weekly time
spent sitting at home while watching TV or VCR, sitting at work or away from
home or while driving, and other sitting at home (eg, reading, meal times,
at desk). They were also asked to report time spent standing or walking around
at home or at work. The responses included 9 categories (ranging from 0 h/wk
to >90 h/wk). In the current analyses, 5 categories were coded consistently
across all the 5 items (0-1, 2-5, 6-20, 21-40, and >40 h/wk).

In 1992, 1994, and 1996, participants were asked the amount of time
they spent on average per week on each of the following physical activities:
walking, jogging, running, bicycling, calisthenics/aerobics/aerobic dance/rowing
machine, lap swimming, squash/racquetball, and tennis. They were also asked
about their usual walking pace, specified as easy/casual (<2 miles per
hour [mph]), normal (2-2.9 mph), brisk (3-3.9 mph), or very brisk/striding
(≥4 mph). From this information, weekly energy expenditure in metabolic
equivalent hours (MET-hours) was calculated.2 For
example, brisk walking requires an energy expenditure of about 4 METs and
is considered to be a moderate-intensity activity. In this cohort, walking
is the most common type of activity (60% of all women reported that they walked
≥1 h/wk). Because the results using baseline and updated physical activity
measures were similar, we reported only baseline analyses.

The reproducibility and validity of the physical activity questionnaire
has been described elsewhere.10 In a representative
sample (n = 147) of participants in the Nurses' Health Study II cohort, the
2-year test-retest correlation for activity was 0.59. The correlation between
physical activity reported on 1-week recalls and that reported on the questionnaire
was 0.79. The correlation between activity reported in diaries and that reported
on the questionnaire was 0.62. In a separate study on a population aged 20
to 59 years recruited from a university community (n = 103), the correlation
between physical activity score on a very similar questionnaire and maximum
oxygen consumption was 0.54.11 In a parallel
cohort of men, average hours of TV watching were significantly associated
with higher levels of leptin and low-density lipoprotein cholesterol and lower
levels of high-density lipoprotein cholesterol and apolipoprotein A1.12

Assessment of Obesity

Body weight was self-reported in the biennial questionnaires. Self-reported
weights were highly correlated with measured weights (r = 0.96; mean difference, 1.5 kg).13 In
1976, the nurses were asked to report their height to the closest inch. Body
mass index (BMI) is calculated as weight in kilograms divided by the square
of height in meters. Incidence of obesity is defined as the transition from
nonobese (BMI <30) in 1992 to a BMI of 30 or more at the end of follow-up
in 1998. Thus, only individuals with a BMI of less than 30 across all time
points (between 1976 and 1992) were included in the obesity analysis (n =
50 277).

Diagnosis of Diabetes

A supplementary questionnaire regarding symptoms, diagnostic tests,
and hypoglycemic therapy was mailed to women who indicated on any biennial
questionnaire that they had been diagnosed with diabetes. A case of diabetes
was considered confirmed if at least 1 of the following was reported on the
supplementary questionnaire: (1) classic symptoms plus elevated glucose levels
(a fasting plasma glucose concentration ≥140 mg/dL [7.7 mmol/L] or a randomly
measured concentration of at least 200 mg/dL [11.1 mmol/L]); (2) at least
2 elevated plasma glucose concentrations on different occasions in the absence
of symptoms (levels as above or ≥200 mg/dL [11.1 mmol/L] after ≥2 hours
of oral glucose tolerance testing); and (3) treatment with oral hypoglycemic
agents or insulin. Our criteria for diabetes classification are consistent
with those proposed by the National Diabetes Data Group.14

The validity of this diagnostic procedure has been verified in a subsample
of this study population.15 In addition, another
substudy assessing the prevalence of undiagnosed diabetes suggested a very
low rate of false-negative results.16

Statistical Analysis

Person-time for each participant was calculated from the date of return
of the 1992 questionnaires to the date of confirmed type 2 diabetes (for diabetes
analysis only), the year of first reported obesity (for obesity analysis only),
death from any cause, or June 1, 1998, whichever came first. Incidence rates
of obesity or type 2 diabetes were obtained by dividing the number of cases
by person-years in each category of average time spent on each sedentary behavior
(eg, TV watching). Relative risks (RRs) were computed as the incidence rate
in a specific category of TV watching divided by that in the reference category,
with adjustment for 5-year age categories. Tests for linear trend across increasing
categories of average time spent watching TV were conducted by treating the
categories as a continuous variable and assigning the middle score for the
category as its value.

We used Cox regression to adjust estimated incidence rate ratios simultaneously
for potential confounding variables. In the multivariate analyses of onset
of obesity, we adjusted for age (<50, 50-54, 55-59, 60-64, ≥65 years),
smoking (never, past, current 1-14, 15-24, and ≥25 cigarettes/d), alcohol
consumption (0, 0.1-4.9, 5-14.9, ≥15 g/d), and physical activity (METs
in quintiles). We further adjusted for dietary variables including total energy
intake, total fat, glycemic load, and cereal fiber (all in quintiles). For
multivariate analyses of diabetes, our covariates included age, smoking, alcohol
consumption, and family history of diabetes. Further analyses adjusted for
dietary intakes of polyunsaturated fat, glycemic load, cereal fiber, and trans fat (all in quintiles).17

We calculated the population-attributable risk to estimate the percentages
of obesity and type 2 diabetes cases in this population that are attributable
to the joint effects of 2 risk factors (either >10 h/wk of TV watching or
<30 min/d walking or equivalent energy expenditure).18P = .05 was considered significant. Statistical analysis
was performed using SAS statistical software, version 8.2 (SAS Institute Inc,
Cary, NC).

RESULTS

Women who spent more time watching TV were more likely to smoke and
drink alcohol and less likely to exercise (Table 1). However, the correlation between TV watching and physical
activity levels was minimal (r = −0.03). These
women also had higher intake of total energy, total and saturated fats, red
meat, processed meat, refined grain products, snacks, sweets/desserts, and
lower intakes of fish, vegetables, fruits, and whole grains.

During 6 years of follow-up (282 141 person years), 3757 women
who were not obese at baseline (7.5%) became obese in 1998. Time spent watching
TV was positively associated with risk of obesity (Table 2). The age-adjusted RRs across categories of TV watching
(0-1, 2-5, 6-20, 21-40, >40 h/wk) were 1.0, 1.23, 1.42, 1.68, and 2.00, respectively
(P for trend, < .001). Further adjustment for
exercise levels and other covariates did not appreciably alter the RRs. Sitting
at work or away from home or driving was also significantly associated with
elevated risk of obesity. In contrast, time spent standing or walking around
at home was associated with a lower risk of obesity (multivariate RR comparing
extreme categories, 0.77; 95% confidence interval [CI], 0.61-0.97; P for trend, <.001). Further adjustment for baseline BMI substantially
attenuated the RRs. The multivariate RR comparing the extreme categories of
TV watching was 1.29 (95% CI, 1.01-1.66). These results suggest that women
who watched more TV were already on a trajectory to become obese at baseline.

During 6 years of follow-up (396 900 person-years), we documented
1515 newly diagnosed cases of type 2 diabetes. After adjustment for age, average
time spent watching TV was significantly associated with increased risk of
type 2 diabetes (Table 3). The
RRs across categories of average hours spent watching TV per week were 1.0,
1.10, 1.30, 1.53, and 1.98 (P for trend, <.001).
Further adjustment for dietary factors slightly attenuated the RRs. As expected,
these RRs were substantially attenuated after further adjustment for BMI (RR
comparing extreme categories, 1.17; 95% CI, 0.82-1.67), suggesting that the
increased risk of diabetes associated with TV watching was largely mediated
through obesity.

Sitting at work and other sitting at home (last category only) were
both associated with significantly increased risk of diabetes in multivariate
analyses adjusting for dietary and nondietary covariates (Table 3). Standing or walking around at home was associated with
a modestly lower risk of diabetes.

To compare the predictive role of various sedentary behaviors and physical
activity, we conducted a multivariate analysis including simultaneously these
behaviors (in continuous form), physical activity (4 METs/d, equivalent to
1 h/d of brisk walking), and nondietary and dietary covariates. Among various
sedentary behaviors, time spent watching TV was most strongly associated with
obesity risk (Figure 1). For each
2-h/d increase in time spent watching TV, there was a 23% (95% CI, 17%-30%)
increase in obesity risk. In contrast, other sitting at home or standing or
walking around home was associated with a significantly lower risk of obesity.
Each 1-h/d increase in brisk walking was associated with a 24% (95% CI, 19%-29%)
reduction in obesity.

Watching TV was also most strongly associated with elevated risk of
type 2 diabetes (Figure 1). Each
2-h/d increment in time spent watching TV was associated with a 14% (95% CI,
5%-23%) increase in diabetes risk. Standing or walking around at home was
associated with a lower risk of diabetes (12% [95% CI, 7%-16%] risk reduction
for each 2-h/d increment). Brisk walking was strongly associated with a decreased
risk of type 2 diabetes (34% [95% CI, 27%-41%] risk reduction for each 1-h/d
increment).

To address the possibility that surveillance for diabetes may have varied
according to physical activity levels, we did an analysis restricted to cases
reporting at least 1 symptom of diabetes at diagnosis. Results from this subgroup
were not appreciably different from those for the entire cohort. The multivariate
RRs across categories of average hours spent watching TV per week were 1.0,
1.09, 1.30, 1.44, and 1.70.

In multivariate analyses, we observed independent effects of TV watching
and exercise levels on obesity and type 2 diabetes (Figure 2). Compared with women who were in the most active (the
highest tertile of METs/wk) and the lowest TV watching category (<6 h/wk
watching TV), those who were in the lowest of METs per week and most sedentary
category (≥20 h/wk watching TV) had a significantly increased risk of obesity
(RR, 1.90; 95% CI, 1.61-2.24) and type 2 diabetes (RR, 2.89; 95% CI, 2.21-3.79).

We estimated that 30% (95% CI, 24%-36%) of new obesity cases and 43%
(95% CI, 32%-52%) of type 2 diabetes cases could be attributable to the joint
effects of 2 risk factors (either >10 h/wk of TV watching or <30 min/d
walking or equivalent energy expenditure). Only 15% of participants in our
cohort belonged to the joint low-risk group (defined as <10 h/wk of TV
watching and ≥30 min/d of walking or equivalent energy expenditure).

COMMENT

In this large prospective cohort of women, sedentary behaviors, especially
TV watching, were significantly associated with risk of obesity and type 2
diabetes. Sedentary occupation reflected by long hours of sitting or standing
at work was significantly associated with risk of obesity. In contrast, even
light activities such as standing or walking around at home (which probably
reflects household work) and brisk walking were associated with a significantly
lower risk of obesity and diabetes. Our study suggests that 30% of obesity
cases and 43% of type 2 diabetes cases can be potentially prevented by following
a relatively active lifestyle (<10 h/wk TV watching and ≥30 min/d of
brisk walking).

Our results are consistent with previous studies of TV watching in relation
to obesity and weight gain in children4,6,7 and
adults.19- 22 They
are also consistent with our previous analysis showing a positive association
between prolonged TV watching and type 2 diabetes among men in the Health
Professionals Follow-up Study.5 Our findings
extend the literature showing that moderate-intensity activity such as brisk
walking is strongly protective against type 2 diabetes15,23- 29 and
indicate a continuum in the relationship between physical activity levels
and obesity and diabetes risk, and TV watching has the highest risk for obesity
and diabetes among several sedentary behaviors.

There are at least 3 potential mechanisms for the observed positive
association between TV watching and obesity and diabetes risk.30 First,
TV watching typically displaces physical activity and thus reduces energy
expenditure.20,21 In our study,
women who spent more time watching TV tended to exercise less, but the effects
of TV watching and exercise on the development of obesity and diabetes were
largely independent. Second, TV watching results in increased food and total
energy intake because individuals tend to eat while watching TV despite their
low physical activity levels.3- 5 Also,
participants who spent more time watching TV tended to follow an unhealthy
eating pattern.5 Such an eating pattern is
directly related to commercial advertisements and food cues appearing on TV31 and has been associated with risk of obesity and
diabetes.32 Another potential mechanism is
that TV viewing results in lower energy expenditure compared with other sedentary
activities such as sewing, reading, writing, and driving a car.2 The
combinations of these factors may explain our findings that TV watching is
more strongly associated with obesity than other sedentary behaviors.

Our findings could have important public health implications. The prevalence
of obesity and type 2 diabetes has increased dramatically in the past several
decades in the United States.33 Although leisure-time
physical activity levels are generally low in the United States, there is
no evidence that recreational physical activity has declined in recent decades.
However, sedentary lifestyle has become more prevalent and pervasive as reflected
by the large numbers of TV sets, VCRs, and remote controls per household and
increasing time spent watching TV in the past several decades.1 We
speculated that increasing sedentary behaviors, especially TV watching, may
have contributed to the obesity epidemic in the United States. Given the strong
relationship observed between sedentary lifestyle and obesity and diabetes
risk, public health campaigns to reduce obesity and diabetes should not only
promote increasing exercise levels, but also decreasing sedentary behaviors,
especially prolonged TV watching. Substantial health benefits can be gained
by even light to moderate activity such as doing household chores and by engaging
in simple and convenient activities such as walking.

The major strengths of this study include its large sample size, prospective
design, and detailed measures of physical activity, sedentary behaviors, and
a multitude of dietary and nondietary covariates. However, the observational
nature of this study cannot prove a causal relationship between TV watching
and obesity because of the potential reciprocal relationship between TV watching
behavior and obesity. This issue could be addressed in randomized clinical
trials among adults. However, school-based intervention studies have already
demonstrated that reductions in TV watching were effective in preventing obesity
among children.8

In conclusion, our data provide strong evidence that sedentary behaviors,
especially prolonged TV watching, are directly related to obesity and diabetes
risk. In contrast, even light- to moderate-intensity activity substantially
lowers the risk. While these findings lend further support to current guidelines
that promote physical activity, they also suggest the importance of reducing
sedentary behaviors in the prevention of obesity and type 2 diabetes.